Chemical Engineering Help With Types And Mechanism Of Corrosion

7.1 Types And Mechanism Of Corrosion

Modern corrosion science has its roots in electro chemistry and metallurgy, where the electro chemistry contributes an understanding of the mechanism that is basic to the corrosion of all metallic objects and the metallurgy provides knowledge of the characteristics of metals and their alloys as well as methods of combining various metals and working them into the desired shapes.
The type of corrosion mechanism and its rate of the attack depend on the exact nature of environment (air, soil, water, seawater ...) in which the corrosion takes place. In today's industrial setting, waste products of various chemical and manufacturing processes find their ways into waterways and air. Many of these substances though often present only in minute amounts, act as either as catalysts or inhibitors for the corrosion process. The corrosion engineer then needs to be on the alert for the effects of these contaminants and apply preventive measures.

Rusting Of Iron
Many of the coatings used to prevent or slow corrosion can have the specific vulnerabilities that need to be understood. The first step in preventing corrosion of material understands its specific mechanism and the second and often more difficult step is designing a type of prevention. Some metals produce corrosion products that are insoluble, about the same size molecularly as the parent metal, and that crystallize in the same type of lattice structure and these are often able to become attached to the metal surface and form a protective coat against further corrosion. The patina formation on copper is an example of this type of coating.

Of all this, of great importance is the conductivity of corroding solution. When large areas of surface are in the contact with a water solution of high conductivity (seawater), the attack on the anodic metal may spread far from its contact point with the cathodic metal and is a less severe situation than that which occurs in soft water or under atmospheric conditions in which the attack is localized in the vicinity of the contact. In the absence of dissolved oxygen or hydrogen ions to maintain the cathode process, galvanic corrosion does not occur and it is possible to combine different metals such as copper and steel in closed hot-water systems with little corrosion.

Other preventive measures involve the use of protective coatings and modification of environment. Some trace impurities (in very less amount) can significantly reduce rate of corrosion and can be added in low concentration to surrounding medium. Paint is the most common coating used to slow the rate of atmospheric corrosion and many other materials, such as plastics, ceramics, rubbers, and even electroplated metals, can be used as protective coatings. The corrosion resistance of a metal can be greatly increased by proper choice of the alloys, for example, aluminum added to brass will increase its corrosion resistance.

Corrosion can be categorized in some common types

uniform corrosion

pitting corrosion

galvanic corrosion

crevice corrosion

concentration cell corrosion

graphitic corrosion

Uniform or General Corrosion

The metal loss is uniform from the surface often combined with high-velocity fluid erosion, with or without abrasives.

Pitting Corrosion

The metal loss is randomly located on the metal surface often combined with stagnant fluid or in areas with low fluid velocity.

Galvanic Corrosion

Occurs when two metals with different electrode potential are connected in a corrosive electrolytic environment. Then the anodic metal develops deep pits and groves in the surface.

Crevice Corrosion

Occurs at places with gaskets, bolts and lap joints where crevice exists and this crevice corrosion creates pits similar to pitting corrosion.

Concentration Cell Corrosion

Occurs where the surface is exposed to an electrolytic environment where the concentration of the corrosive fluid or the dissolved oxygen varies and is often combined with stagnant fluid or in areas with low fluid velocity.

Graphitic Corrosion

Cast iron loosing iron in salt water or acids and leaves the graphite in place, resulting in a soft weak metal.

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